(1) Field of the Invention
The present invention relates to an improvement in an electric control apparatus of a cage type induction motor.
(2) Description of the Prior Art
Recently, there has been developed and utilized an electric control apparatus for an induction motor, that is a so-called field oriented control apparatus, in which the exciting current and the secondary current are independently controlled to control the amplitude, frequency and phase angle of the primary current, whereby the generating torque is adjusted. According to this control apparatus, the exciting current of the induction motor and the secondary current, which has a direct relation to speed control of the induction motor, can be controlled independently, and therefore, the speed response characteristic of the induction motor can be improved to a level comparable to that of a DC motor.
As an example of the field oriented control apparatus of this type, reference can be made, for example, to the constant flux and slip frequency control type vector control apparatus shown in FIG. 6 attached to the treatise entitled "Speed Control of Induction Motor" which was published at the symposium of the national meeting of the Japanese Electric Association in April 1979.
This known field oriented control apparatus still involves the following problem.
The resistance value of a secondary resistor of a cage type induction motor is remarkably influenced by such factors as the load and the ambient temperature. If the resistance value of the secondary resistor is considerably changed, as described hereinafter, the slip frequency of the induction motor is not controlled to a correct value proportional to the secondary current preset by the vector control apparatus, with the result that problems relating to variations in the voltage and torque are brought about in the induction motor.